1. Field of the Invention
The present invention relates to a process for preparing an .alpha.-hydroxycarboxylate. More specifically, the present invention relates to a process for efficiently preparing, from an .alpha.-hydroxycarboxylic amide and an alcohol in an industrially advantageous manner, an .alpha.-hydroxycarboxylate which is useful as any of raw materials for solvents, food additives, perfumes, medicines, agricultural chemicals, industrial chemicals, degradable polymers and .alpha.-amino acids.
2. Description of the Prior Art
It is known that .alpha.-hydroxycarboxylates are industrially extremely important compounds, and for example, lactates which are one kind of .alpha.-hydroxycarboxylates are used as high-boiling solvents and as raw materials for food additives, perfumes, medicines, agricultural chemicals and degradable polymers. In addition, the .alpha.-hydroxycarboxylates can be used as solvents, as raw materials for the production of methacrylates, particularly methyl methacrylate by dehydration, and as raw materials for the production of .alpha.-amino acids by aminolysis.
As techniques for manufacturing the .alpha.-hydroxycarboxylates, some methods are known from former days which comprise reacting an alcohol with a nitrile in the presence of an acid catalyst. For example, as preparation techniques of lactates, Japanese Patent Publication Nos. 8061/1955 and 2333/1965 disclose methods which comprise dissolving lactonitrile in an alcohol and water, adding sulfuric acid to the solution to carry out hydrolysis and esterification, and then introducing an alcohol vapor into the resultant mixture. With regard to .alpha.-hydroxyisobutyrates, there are known methods for preparing them by reacting acetone cyanohydrin with an alcohol in the presence of an acid catalyst, and these methods are disclosed in, for example, U.S. Pat. No. 2,041,820 and Japanese Patent Application Laid-Open No. 230241/1992. These reactions are exothermic and can spontaneously proceed, and for the reason, a manufacturing apparatus is simpler as compared with other methods which will hereinafter be described. However, since the acid catalyst is used, an anticorrosive material is required to be used, and what is worse, a large amount of a salt is disadvantageously produced as a waste product.
As techniques for the manufacture of the .alpha.-hydroxycarboxylates, there are methods which comprise reacting an .alpha.-hydroxycarboxylic amide with an ester. For example, Japanese Patent Application Laid-Open Nos. 178792/1993 and 145106/1994 disclose methods which comprise reacting .alpha.-hydroxyisobutyric amide with a formate to obtain an .alpha.-hydroxyisobutyrate. This reaction is an exchange reaction between the amide and the ester and hence it is neither noticeably exothermic nor endothermic, so that any anticorrosive material is not necessary. However, these techniques have a drawback that the reaction proceeds merely until an equilibrium composition of the amide and the ester has been reached.
Furthermore, known is a method which comprises directly reacting an .alpha.-hydroxycarboxylic amide with an alcohol to prepare the .alpha.-hydroxycarboxylate. This method is characterized in that any salt to be discarded is not produced, and any upper limit of a conversion of the raw material amide is not present in principle. For example, Japanese Patent Application Laid-Open No. 3015/1977 describes a method in which the reaction is carried out in the presence of a metal carboxylate, while the reaction system is intermittently purged with a gas. In this method, however, a yield is low and a large amount of by-products is formed, and for these reasons, the disclosed method is not practical.
In addition, Japanese Patent Application Laid-Open Nos. 345692/1994, 258154/1995 and 73408/1996 suggest methods which comprise performing a reaction in the presence of an insoluble solid acid catalyst or a metallic catalyst, while a large amount of a nitrogen gas is fed and generated ammonia is discharged. According to investigation by the present inventors, it has been apparent that in order to prepare the .alpha.-hydroxycarboxylate in a high yield in accordance with each of these methods, a large amount of nitrogen is necessary. Thus, for the sake of the collection of ammonia from the discharged gas, a good deal of energy is required. Furthermore, the amount of the catalyst to be used increases, a long period of time is required for the reaction, and a large amount of by-products is formed. Accordingly, the disclosed methods are not industrially practical.